This invention relates to a thickness vibration mode piezoelectric vibrating element. By the term thickness vibration mode is meant thickness shear, thickness twist and thickness extensional modes.
A double mode coupled quartz vibration element shown in FIG. 1 is old, for example U.S. Pat. No. 3,401,276. A crystal (quartz) plate 1 is produced by cutting a quartz crystal in parallel with the X-Z' plane of crystalline axes X, Y', and Z' in which axes Y' and Z' correspond to Y and Z' axes which are respectively rotated by a predetermined angle (for example 35.degree.15' in the case of AT cut) about the X axis. Common exciting electrodes 2a and 2b are applied to one surface (front surface) of the crystal plate 1, and divided exciting electrodes 3 and 4 are applied to the opposite or rear surface. These electrodes are formed by vacuum deposition of such electroconductive metals as gold, silver and aluminum. Electrodes 2a and 3 and electrodes 2b and 4 oppose each other through the crystal plate 1. The common electrodes 2a and 2b are connected to a lead wire 6 via connecting electrodes 5a and 5b respectively, whereas the divided electrodes 3 and 4 are connected to lead wires 9 and 10 respectively through connecting electrodes 7 and 8.
Vibration adjusting members 11 and 12 are formed on the electrodes 2a and 2b for the proposed of adjusting the vibration frequency of the vibrating member as will be described later. The vibration adjusting members are prepared by vapor depositing the same material as said electrodes or magnesium fluoride (MgF.sub.2).
Electrodes 2a, 2b, 3, 4, 5a, 5b, 7 and 8 are formed by primary vapor deposition, and at a stage of forming these electrodes, the vibration adjusting members 11 and 12, shown by shaded areas, are not yet deposited. These vibration adjusting members 11 and 12 are formed on the electrodes 2a and 2b by the following method. More particularly, the output level characteristic of the crystal plate subjected to the primary vapor deposition is measured by a measuring circuit shown in FIG. 2, which is usually used to measure the output of a piezoelectric vibrating element and comprises an oscillator 13, resistors 14 and 15 and a detector 16. The result of measurement is generally shown by a solid line curve a shown in FIG. 3. Since it is difficult to match the center frequency f.sub.01 of the primary vapor deposited portions to a desired center frequency f.sub.0 (for example 10.7 MHz) the center frequency f.sub.01 is set to a value higher than the desired center frequency f.sub.0 by a percentage of frequency variation of 10.sup.-4 -10.sup.-3. The vibration adjusting members 11 and 12 are provided for the purpose of causing the center frequency of f.sub.01 of the crystal plate 1 obtained by the primary vapor deposition to approach the desired center frequency f.sub.0 so that suitable amounts of the vibration adjusting members are formed on the electrodes 2a and 2b by secondary vapor deposition by using the result of measurement described above.
In this manner, a vibrating element having a desired center frequency can be obtained. However, the output level characteristic of the vibrating element deposited with the vibration adjusting members 11 and 12 changes from curve a to curve b shown in FIG. 3. In other words, curve b shows the characteristic obtainable when the center frequency is matched with that of the crystal plate subjected to the primary vapor deposition. Curve b shows that, at a frequency about 100KHz higher than the center frequency f.sub.0, attenuation of about -40 dB caused by the primary vapor deposition decreases to about -20 dB whereby undesirable side resonance tends to occur.
Such undesirable side resonance caused by the secondary vapor deposition was also noted in a multi-mode coupled crystal vibrating element comprising more than three pairs of exciting electrodes, and a crystal vibrating element provided with a pair of exciting electrodes and more than two pairs of exciting electrodes which are not mode-coupled. However, any means has not been available which is effective to prevent such undesired side resonance.